It is often desirable to have ceramic powders of high quality suitable for firing into formed articles which contain little or no impurities and are of a predetermined particle size, whether monosized or of a particular size distribution. Preferably, such powders are substantially free of agglomerates. Such powders, for example aluminum nitride, are useable as electronic substrates upon forming and sintering. Preparation of substantially pure ceramic powders is difficult, to say the least, and has been the subject matter of much experimentation.
U.S. Pat. No. 4,618,592 to Kuramato et al. discloses several methods of preparing aluminum nitride powder. One method, known as direct nitriding, includes nitriding metallic aluminum powder at high temperatures in an atmosphere of nitrogen or ammonia and pulverizing the resulting nitride. Since metallic aluminum is used as a starting material, it naturally requires a step of pulverizing metallic aluminum at some point in order to achieve a reasonable rate of nitridation. In addition, in order to improve sinterability of the resulting nitrides produced by direct nitriding, the nitrides typically require further pulverizing, which may introduce impurities into the material, e.g. during ball-milling oxidation may take place, as well as contamination by way of degradation of the mechanical apparatus. Finally, with respect to direct nitriding, there is inherent in the process several percent by weight of residual aluminum metal which is undesirable.
Another conventional method of producing aluminum nitride is to mix dry powders of alumina and carbon and carbothermally reduce the alumina. Here again, pulverizing is generally required at several points in the process and purity is typically a problem.
A third known method of producing aluminum nitride includes wet mixing powders of alumina and carbon in a liquid dispersing medium, carbothermally reducing the alumina in the mixture, followed by heating in air or other oxidizing atmosphere to remove unreacted carbon. The resulting nitride powder is generally superior to the product of the above-mentioned processes, however, there are still relatively high levels of agglomerates and impurities. FIG. 1 is a photomicrograph of a powder produced in accordance with this latter process, which is described in the '592 patent referred to hereinabove, and further described in Example 1 hereof.
It is accordingly an object of the invention to produce fine powders from oxidized precursors by reducing them in the presence of carbon to achieve powders with low levels of agglomerates.
It is a further object of the invention to produce powders from their corresponding oxides by carbothermal reduction with a minimum of impurities.
Another object of the invention is to produce powders of controlled particle size which may be altered or changed through selection of appropriate precursor particles.
Another object of the invention is to produce powders which are substantially free of aggregates, or which contain only loosely agglomerated primary particles which are easily separated or dispersed with further processing.
Still further objects and advantages of the present invention will become readily apparent upon consideration of the following description, figures and claims.